Optical fibers efficiently transmit information at high rates and long distances. These fibers are delicate and need to be protected. Conventionally, one or more optical fibers are incorporated into a fiber optic cable that protects the fibers from mechanical damage and/or adverse environmental conditions such as moisture exposure. Examples of protective components include extruded buffer tubes, core tubes and slotted core members.
A typical construction of a loose buffer tube optical cable, a common optic cable design, is described in US 2006/0045439 (Brown et al; The Dow Chemical Company). In brief, the optic cable is structured with buffer tubes positioned radially around a central strength member and wrapped in a helical rotation in the axial length. The arrangement of the buffer tubes in a helical rotation allows bending of the cable without significant stretching of the tube or optic fibers contained within. The buffer tubes are typically filled with optic cable grease incorporating hydrocarbon oil surrounding the optical fibers and eliminating air space. The grease provides a barrier against water penetration, which can be detrimental to the optic transmission performance. If a reduced number of buffer tubes are used, one or more foamed filler rods can be used as low cost spacers to occupy one or more buffer tube positions to maintain cable geometry. Typically, a water blocking functionality is incorporated into the cable core via use of components such as yarns or core wraps that incorporate water-swellable super absorbent polymers. The elements are surrounded within a jacket, which is typically composed of a polyethylene.
Design elements for loose buffer tube cables can vary, for example, according to the size and materials of construction for the central strength and tensile member, the dimensions and number of buffer tubes, and the use of metallic armors and multiple layers of jacketing material. Other components such as water blocking treatments on the central strength member, or ripcords to aid removal of jacketing for installation are also common elements. Another variation is to eliminate the buffer tube grease and employ super-absorbent water-blocking functionality such as yarns or powders within the buffer tubes.
US 2006/0045439 also describes a typical optic cable incorporating a core tube (also known as a “central tube”). In brief, the optic fibers are positioned near the center of the cable within a central, cylindrical core tube. The optical fibers are in bundles embedded in a filling material surrounded by a cylindrical core tube. Ripcords, situated on the surface of the core tube, are surrounded by water blocking tape. A corrugated coated steel cylinder surrounds the tape to protect the optic fiber bundles. Wire strength members provide the cable with strength and stiffness. The components are surrounded by a jacket, which is typically composed of polyethylene. In the described design, all of the mechanical functions are incorporated into the outer sheathing system composed of the core tube, polyolefin jacketing layers, tensile and compressive strength members, metallic armors, core wraps, water blocking components and other components. The core tube is typically larger in diameter than a buffer tube to accommodate the bundles of optic fibers or ribbon components containing the optic fibers. A core tube typically contains a water blocking grease surrounding the optic fiber components, although dry designs incorporating super-absorbent polymer elements for water-blocking can be used. The optimal material characteristics for a core tube component are similar to those of a buffer tube application.
US 2006/0045439 further describes an embodiment of an optical cable which incorporates a slotted core tube. In brief, the slotted core tube has a central member to prevent buckling and control shrinkage of the extruded slotted core profile shape. The slotted core tube includes slots in which optical fibers are positioned. A filler rod can optionally occupy one or more slots. The slotted core is surrounded by a water blocking layer which can include one or more ripcords. The water blocking layer is surrounded by a dielectric strength member layer, which in turn is surrounded by a jacket typically composed of polyethylene.
Optical cables are generally manufactured using high modulus materials to provide the cable and extruded optical cable protective components (e.g., buffer tubes, core tubes and slotted core tubes) with good crush strength. Extruded optical cable protective components are typically filled with hydrocarbon-based greases (also referred to as “gels”) that provide a water-blocking function. These greases typically contain low molecular weight hydrocarbon oils that can be absorbed into the polymeric tube materials, adversely affecting mechanical properties such as decreased flexural modulus and crush resistance. A decrease in crush resistance can compromise optic cable performance by making the optic fibers more prone to mechanical stress resulting in an increase in signal attenuation. In addition, under several application conditions, the loss of crush resistance increases the possibility of catastrophic failure via mechanical damage to the optic fibers. Thus, minimal oil absorption with good retention of flexural modulus and crush resistance, commonly referred to as “grease compatibility,” is an important performance characteristic for polymer materials to be used for the extruded optical cable protective components.
Different polymeric materials have shown different grease (gel) absorption characteristics. For example, polybutylene terephthalate (PBT) has shown only minimal change in physical properties following conditioning in optic grease, whereas polyolefin polymers have shown greater changes in properties. Higher crystalline polyolefin materials have typically shown a much reduced change in properties compared to more amorphous materials, and impact modified polypropylene (IMPP) is very prone to grease absorption.
Another important performance parameter for extruded optical cable protective components is post-extrusion shrinkage characteristics. When extruded optical cable protective components containing optical fibers are fabricated, it is important to the optical signal transmission that the optic fibers do not have excess slack, referred to as “excess fiber length” or EFL. Rapid shrinkage of an optical cable protective component which occurs during extrusion processing typically does not contribute to EFL because the optic fibers are moderately tensioned during the process. However, post-extrusion shrinkage of an extruded protective component (e.g., buffer tube) can result in EFL for the contained optical fibers causing the fibers to extend beyond the ends of the protective component, leading to stresses on the optic fibers and attenuation of the signal.
Such shrinkage can occur late in the fabrication process after the fiber tensioning capability has been overcome by frictional forces or following manufacture of the component. Two primary mechanisms for shrinkage of an extruded optical cable protective component are strain recovery of the viscoelastic stretching of the polymeric melt during the tube shaping extrusion process, and solid state annealing shrinkage resulting from a continuing re-crystallization of the polymeric matrix. To provide an optical cable protective component that exhibits low post extrusion shrinkage and a low EFL, it is desirable to use materials that can provide a fast relaxation of viscoelastic melt stresses and minimize subsequent annealing shrinkage.
Polybutylene terephthalate (PBT) is often used for buffer tube applications due to performance attributes of high stiffness and deformation resistance (with flexural modulus>2,400 MPa) and low EFL caused by post-extrusion shrinkage. However, PBT is relatively expensive, especially on a cost per volume basis, compared to polyolefin-based compounds.
There has also been use of lower cost polyolefin materials such as high density polyethylene (HDPE) and impact modified polypropylene, in both buffer tube and core tube applications. However, because HDPE has a lower modulus, a lower crush resistance and an increased level of post extrusion shrinkage compared to materials such a polypropylene (PP) and PBT, special fabrication care is required to avoid high levels of EFL that are detrimental to signal attenuation performance. In addition, although HDPE provides a higher level of optic grease compatibility compared to IMPP, both IMPP and HDPE have substantially lower modulus and crush resistance than PBT, especially after grease exposure. Consequently, the use of HDPE has been limited in replacing PBT or PP in buffer tube applications.
It would be desirable to provide a material based on HDPE that can be used in fabricating extruded optical cable protective components having reduced shrinkage and EFL for use in fiber optic cables.